Chopped glass fibers are commonly used as reinforcement materials in thermoplastic articles. Typically such fibers are formed by drawing molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film-forming binder resins to the filaments, gathering the filaments into strands, chopping the fiber strands into segments of the desired length and drying the sizing composition. These chopped strand segments are thereafter mixed with a polymerized resin and the mixture supplied to a compression or injection molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped strands are mixed with dried powder of a polymerized thermoplastic resin, and the mixture supplied to an extruder wherein the resin is melted, the integrity of the glass fiber strands is destroyed and the fibers are dispersed throughout the molten resin, and the fiber/resin mixture is formed into pellets. These pellets are then fed to the molding machine and formed into molded articles having a substantially homogeneous dispersion of the glass fibers throughout.
Alternatively, fiber reinforced thermoplastic composites may be formed from compression molding of fibrous mats laden with thermoplastic polymer. Methods of making such fiber reinforced composite materials from an aqueous slurry of solid polymer and reinforcing material are known. See Published European Patent Applications 0,148,760 and 0,148,761, Wessling et al., U.S. Pat. No. 4,426,470 issued Jan. 17, 1984 and Gatward et al., U.S. Pat. No. 3,716,449 issued Feb. 13, 1973, all of which are incorporated herein by reference. In general, these reinforced polymer composites have a uniform mixture of fiber, polymer and binder, and are prepared from dilute aqueous slurries of a solid heat-fusible organic polymer, a reinforcing material and optionally a latex binder. Wessling et al. U.S. Pat. No. 4,426,470 issued Jan. 17, 1984 discloses on column 4, lines 18-21 that various chemical additives such as antioxidants, UV stabilizers, thickeners, foaming agents, antifoaming agents, bactericides, electromagnetic radiation absorption agents, etc., may also be used in the composites comprising a heat fusible polymer and reinforcing material.
Alternatively, sections of a preformed glass mat, or other shaped glass mat, for example U-shaped channel, or chair seats, may be impregnated with a thermoplastic resin powder, and then thermoformed under sufficient heat and pressure to melt the thermoplastic polymer and bond the glass mat. For PVC impregnated glass mats, the PVC is typically dry mixed with a thermal stabilizer and alpha-SAN, and any other additives, to form a homogenous powder prior to impregnation of the glass mat. If the glass mat is to be impregnated with an impact modified blend, the impact modifier is typically added as a powder and dry-mixed with the other ingredients and does not interfere with formation of the single phase of PVC and alpha-SAN. Thereafter, the glass mat is then `dusted` or `filled` with the desired amount of the powder mix, generally so that there is from about 30% to about 60% mix evenly spread through the mat, and the dusted mat is then molded under pressures of from 100-1000 psi and temperatures of from 170-190.degree. C., to form the shaped glass fiber reinforced PVC blend article.
Glass mat, or other shaped glass fiber stock may also be impregnated with a melt of the blend ingredients, such as in pultrusion. Typically, there is about an equal weight of resin and glass fibers in each sheet. Several such sheets may be cut to a predetermined configuration, stacked in a mold and conventionally molded at a temperature of 160-200.degree. C. and a pressure of about 1000 psi (about 30,000 lbf) to form a thick-walled shaped article.
Although there are numerous methods for making reinforced composite molding materials, many of these processes are either too inefficient, or cannot be sufficiently controlled to produce a fiber reinforced product that provides the resulting composite article with sufficient properties, such as strength, to meet performance requirements. Thus, even with the current technology of aqueous methods for making reinforced composite materials, there are numerous drawbacks, including the loss of polymer properties as a result of the manufacturing process. Specifically, in the previous technology when using solution polymers such as polyvinyl chloride, the vinyl chloride is polymerized, stripped of residual free monomer, the polymer is then processed by some combination of centrifugation, filteration, and drying. Additives are usually blended at this point. Typically, the dry polymer is combined with the reinforcement either by extrusion, dry blending, or aqueous methods. However, once compounded, the PVC has a significant heat history since prior to it being combined with reinforcement it has already been heated in order to be dried. Then there is the subsequent heating that occurs while being combined with the reinforcement. As a result, prior to the compound ever being molded to a final product, a number of important properties have already been reduced by the multiple applications of heat to the polymer.
Accordingly, a need exists for a more efficient process that controllably yields a fiber reinforced molding compound that provides enhanced performance characteristics to the composite articles molded therefrom. Such a process would preferably eliminate the need for binder and improve the heat history of the polymer. This need is fulfilled by the process of the invention described below.
In addition to the above drawbacks, the current injection molding technology does not have the ability to retain the length of the reinforcement material. In particular, the present technology for combining reinforcement and polymer results in a compound that has the consistency of sand, thus obliterating the length of the reinforcement. The invention described below has the ability to retain reinforcement length. For example, if a 11/4" chopped glass fiber is used, the final composite contains reinforcement essentially of that length. The same is true for using continuous reinforcement.